1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, to a semiconductor device having a plurality of power-supply nodes.
2. Description of the Background Art
Today, further reduction in power consumption in a semiconductor device is required. Particularly, a low voltage, low power operation in a stand-by mode is essential in a device such as portable equipment that is battery-driven.
As an example, let us consider a semiconductor device driven by two power supplies in which power-supply voltages of 3.3V and 2.5V, for example, are provided from outside. In this case, as a way of reducing the power consumption, the semiconductor device is formed such that an internal circuit that is not in operation in a stand-by mode is provided with a power supply of 2.5V, while a circuit that is required to be in operation even in the stand-by mode is supplied with 3.3V. Thus, the power consumption in a stand-by mode can be reduced by having the supply of 2.5V power supply potential stopped in the stand-by mode.
When stopping the supply of 2.5V power-supply potential, however, if the state of each node for the internal circuit driven by 2.5V is not taken into account, an input node for the internal circuit of the 3.3V system operating in the stand-by mode may enter a floating state, or the indefinite state, thereby causing an erroneous operation of the semiconductor device. Especially in a CMOS circuitry, there are many circuits with an arrangement that allows a large through current to flow steadily when an input node reaches an intermediate potential.
As a solution to this problem, a detection signal indicating that the supply of 2.5V power-supply potential is stopped may be provided from outside, and an internal circuit that operates according to this signal and provides a fixed potential to a node may also be provided so as to fix the potential of the node that needs to be fixed in order to prevent an erroneous operation.
The power consumption of the entire system in a stand-by mode, however, is not readily reduced when a circuit for detecting a power-supply potential is provided outside the semiconductor device, because a detection circuit needs to be in operation ready for the next power-on even while the supply of 2.5V power-supply potential is stopped.
An object of the present invention is to provide a semiconductor device capable of recognizing a power-off without relying on an external signal, and fixing the potential of a node that needs to be fixed, thereby preventing an erroneous operation when the supply of one of the two systems of power-supply potentials is stopped.
In short, the present invention is a semiconductor device provided with a first power-supply line, a first internal circuit, a second power-supply line, and a second internal circuit.
The first power-supply line is provided to transmit a first power-supply potential. The first internal circuit receives and operates by the first power-supply potential from the first power-supply line to output a first main signal to a first internal node. The second power-supply line is provided to transmit a second power-supply potential. The second internal circuit receives and operates by a second power-supply potential from the second power-supply line. The second internal circuit includes a third internal circuit that monitors the potential of the first power-supply line and outputs a first sub signal that decides the potential of the first internal node instead of the first main signal that becomes indefinite upon the detection of a power-off.
Thus, the main advantage of the present invention is that problems such as an erroneous operation and data disruption can be prevented while a stable operation is ensured, since the semiconductor device itself can recognize the power-off without relying on an external signal and provides a potential to the node that becomes unstable when the supply of one of the two systems of power-supply potentials is stopped.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.